code: def test_pandas_univariate_dateindex():
      # Impulse responses still have RangeIndex (i.e. aren't wrapped with dates)
      ix = pd.date_range(start="2000", periods=1, freq=MONTH_END)
      endog = pd.Series(np.zeros(1), index=ix)
      mod = sarimax.SARIMAX(endog)
      res = mod.filter([0.5, 1.0])
  
      actual = res.impulse_responses(2)
      desired = pd.Series([1.0, 0.5, 0.25])
      assert_allclose(res.impulse_responses(2), desired)
      assert_(actual.index.equals(desired.index))
  
imports: from statsmodels.compat.pandas import MONTH_END
  import warnings
  import numpy
  from numpy.testing import assert_, assert_allclose
  import pandas
  import pytest
  from scipy.stats import ortho_group
  from statsmodels.tools.sm_exceptions import EstimationWarning
  from statsmodels.tsa.statespace import dynamic_factor, mlemodel, sarimax, structural, varmax
  from statsmodels.tsa.vector_ar.tests.test_var import get_macrodata
label_desc: Use image node_id 12 for calling a global function with example usage: test_pandas_univariate_dateindex() without return types